In a vehicle described in Patent Literature 1, as shown in FIG. 25, front wheels are designed to be driven by a main drive source such as an engine (both the wheels and the engine being not shown), and rear wheels 102 of the vehicle 100 are designed to be driven by a motor 103, which is an auxiliary drive source, via a power transmission mechanism 104.
This power transmission mechanism 104 includes a reduction gear mechanism 105 into which power from the motor 103 is inputted and a differential gear 106 that distributes the power outputted from the reduction gear mechanism 105 to the left and right rear wheels 102, 102. The reduction gear mechanism 105 includes a reduction gear train that is made up of a first gear 105a that is fixed to an output shaft of the motor 103, a second gear 105b that meshes with the first gear 105a and a third gear 105c that meshes with an input gear 106a of the differential gear 106. A hydraulic clutch 107 is provided between the second gear 105b and the third gear 105c. When the hydraulic clutch 107 is engaged, the second gear 105b and the third gear 105c are coupled together, whereby the power of the motor 103 can be transmitted to the rear wheels 102 via the power transmission mechanism 104. When the hydraulic clutch 107 is disengaged, the second gear 105b and the third gear 105c are decoupled from each other, whereby the transmission to the rear wheels 102 of power of the motor 103 is disconnected.